1. Insert OS X CD that came with MacBook;
2. Reboot;
3. Hold down C key when start-up chime plays;
4. Hold C key down until screens turns blue and you see a progress bar;
5. Once fully booted, navigate to "Install" menu > Disk utility;
6. Highlight you hard-drive (the drive and not any of its partitions);
7. Go to "Initialise" tab and hit Initialise.
This will completely wipe you hard-drive clean; you will loose any data you have not backed-up to a different medium/drive as well as all partitions.
8. Once initialisation is done, proceed to re-install OS X.
These 8 steps will give a MacBook as it would be straight from Apple.
Hope this helps. 🙂